1. Field of the Invention
The present invention generally relates to the art of wireless communications system. In particular, the present invention relates to the art of minimizing power consumption in a time-division access system mobile station.
2. Description of Related Art
A. Basic GSM System Architecture.
As reflected in FIG. 1, a GSM cellular network is comprised of three fundamental parts:
(1) a mobile station 1 (which is carried by the subscriber);
(2) a base station subsystem 2 (which controls the radio link with the mobile station); and
(3) a network subsystem 3 (which is interfaced to the public fixed network 4 and the base station subsystem).
The network subsystem and the base station subsystem communicate across the "A" interface 5, while the mobile station and the base station subsystem communicate across the air interface or radio link (also called the "Um" interface) 6.
It should be noted, however, that application of the present invention is not necessarily limited to use within a GSM system, but can also be applied to the mobile stations of other wireless communications systems.
Mobile Station. The mobile station is the "phone" part of the wireless communication system. The mobile station may be fixed or portable. Fixed mobile stations are permanently installed in a car or a stationary location. Portable units include bag phones and hand-portable phones (commonly called "cell phones"). Hand-portable phones are becoming increasingly popular because they can he carried easily on the person of the subscriber. A mobile station includes an antenna 7 for transmitting and receiving radio signals from the base station subsystem.
Base Station Subsystem. The base station subsystem comprises two fundamental elements, (1) one or more base transceiver stations (8 and 9) and (2) a base station controller 10. These components communicate across the Abis interface 11. A base transceiver station includes radio transceivers that handle radio-link protocols with the mobile station and an antenna 12 for communication with mobile stations.
The base station controller manages the radio resources of the base transceiver stations. It also manages handovers (passing the audio from cell to cell during a call), frequency hopping (changing operating frequency to maintain signal quality) and radio-channel setup.
Network Subsystem. The basic element of the network subsystem is the mobile services switching center (MSC) 13. The MSC is the interface of the cellular network to the public fixed network and, as such, basically performs the functions of a switching node of the public fixed network. The MSC also routes calls from the public fixed network (via the base station controller and the base transceiver station) to the mobile station. The MSC also provides the GSM system with individual information about the various mobile stations and performs the functions of authentication, location updating, and registration. The MSC may operate in conjunction with other functional entities which further comprise a network subsystem, such as registers which hold information regarding current mobile station location and subscriber information.
B. Air Interface or Um Interface.
In conventional wireless communications technology, user data (e.g. speech) is encoded in a radio frequency for transmission and reception between a base station and a mobile unit. Because the number of available radio frequencies, or "channels," for cellular system is less than the number of all possible users, the system is "trunked." Trunking is the process whereby users share a limited number of channels in some predetermined manner.
A common form of trunked access is the frequency-division multiple access (FDMA) system. In FDMA, the limited channels are shared by all users as needed. However, once a channel is assigned to a user, the channel is used exclusively by the user until the user no longer needs the channel. This limits the number of concurrent users of each channel to one, and the total number of users of the entire system, at any instant, to the number of available channels.
Another common trunking system is the time-division multiple access (TDMA) system. TDMA is commonly used in telephone networks, especially in cellular telephone systems, in combination with an FDMA structure. In TDMA, data (speech) is digitized and compressed to eliminate redundancy and silent periods, thus decreasing the amount of data which is required to be transmitted and received for the same amount of information. Each of the channels used by the TDMA system is divided into "frames" and each of the users sharing the common channel is assigned a time slot within the frames. The TDMA system appears, to each of the users sharing the channel, to have provided an entire channel to each user.
A FDMA and TDMA combination technique is used by the GSM system. In GSM, each channel is divided up in time into frames during which eight different users share the channel. A GSM time slot is only 577 .mu.s (microseconds), and each users gets to use the channel for 577 .mu.s every 4.615 ms (milliseconds). 577 .mu.s*=4.615 ms. Transmission is achieved in bursts.
C. Mobile Station Architecture.
As shown in FIG. 2, mobile stations generally comprise two basic parts, the RF (radio frequency) part 20 and the digital part (or baseband processing circuitry) 21. The RF part operates receiving, transmitting, and modulation functions. The digital part handles data processing, control, and signaling functions. As shown, the radio frequency part includes an antenna 27 for receiving and transmitting radio signals. A radio signal received by the radio frequency part is converted to a lower frequency signal and delivered 22 to the digital part. Likewise, a signal generated by the digital part is delivered 23 to the radio frequency part, which in turn converts the signal to a higher frequency signal, and transmits that higher frequency signal.
The digital part is operatively connected to a handset 24, which has a speaker 25 and a mouthpiece 26. All or part of the radio frequency part and the digital part can be disposed within the handset, as is the case with cell phones.
Receiving Architecture. As shown in FIG. 3, a mobile station comprises an antenna 50 for transmitting and receiving radio signals. In FIG. 3, the antenna is coupled to both receiving 51 and transmitting 52 paths. The receiving path is connected to a receiver 53. The receiver contains a receiving filter network and a mixer to convert the input signal into an intermediate (lower) frequency. The down-converted signal is converted into a digital signal by an analog-to-digital converter 54. An equalizer adjusts for distortions 55. A demodulator 56 extracts the bit stream from the intermediate frequency, and a demultiplexer 57 sorts the information from the different time slots and frames into their appropriate individual logical channels.
It should be noted that while a GSM mobile station comprises an equalizer, there exist other types of mobile stations that do not have an equalizer function, such as CDMA mobile stations. As would be recognized by someone skilled in the relevant art, some of the other details described herein are GSM specific and may be different in another wireless communications system. This fact should not be read to limit the present invention only to GSM applications.
The channel codec 58 decodes the bit sequence coming from the demultiplexer. The channel codec passes a signaling frame to the control processor 59 and passes a speech frame to a speech codec 60.
The speech codec rebuilds sounds out of 260-bit speech blocks received from the channel codec and passes the digitized speech to the digital-to-analog converter 61. The analog signal is converted to sound by an ear piece 62 of the phone or handset 63. The control processor performs the control functions of the mobile station, including, for example, power control and the selection of different channels.
Also included in the mobile station architecture is a reference clock, which is used to drive the digital hardware. In a GSM system, the reference clock is required to run at 13 MHz within 0.1 ppm while the mobile station is in active use.
Clock circuitry may also include tuning circuitry or temperature compensation circuitry to make the reference signal more accurate. For purposes of interpreting the claims herein, "clock" should be interpreted to include such circuitry where the function of the circuitry is to increase the accuracy of the signal with respect to the desired frequency.
Transmitting Architecture. Voice is received via the mouthpiece 70 of the phone and converted to an electrical analog signal. The analog signal is then converted to a digital signal by an analog-to-digital converter. The speech codec compresses the digitized speech coming from the analog-to-digital converter 71 such that the data are represented by 260 bit blocks before being encoded. The channel codec encodes each bit sequence before going to a multiplexer. The burst-building unit 72 places the coded bits in the transmitter path into the appropriate burst structure. The multiplexer 73 assigns each burst to a time slot within a numbered frame in which it should be transmitted. After this sorting and ordering has been done, the modulator 74 imparts this information on the intermediate frequency carrier.
The digitized signal passes through a digital-to-analog converter 75 and the transmitter 76 up-converts the modulated intermediate frequency signals from the modulator to the final radio frequency in the 900-MHz band. The signal is transmitted via the antenna.
The hardware for converting analog signals to digital signals, and visa versa, (i.e., FIG. 3, items 54, 61, 71 and 75) may be called "mixed signal." Also, the equalizer, demodulator, demultiplexer, channel codec, speech codec, burst building unit, multiplexer and modulator functions described above (i.e., FIG. 3, items 55, 56, 57, 58, 6(), 72, 73 and 74) can be handled by (in whole or in part) a digital signal processor ("DSP"). The operation of the DSP can be made more efficient with appropriate hardware accelerators. The RF part in FIG. 3 comprises items 51, 52, 53 and 76.
A block diagram of a mobile station design with a DSP is shown in FIG. 4. Shown in FIG. 4 is the microphone 100 and speaker 101, which are operationally connected to the mixed signal 102. The radio frequency circuitry 103 is also operationally connected to the mixed signal. There is also interface logic 104 which interfaces with the radio frequency circuitry, and the mixed signal. Further, the interface logic interfaces with the mobile station keypad 105, the mobile station display 106, a subscriber identity module (SIM) card (which provides the mobile station with identifying information) 107, a serial port 108, and a battery 109. The control processor 110 and control processor program/data memory 111 also interface with the interface logic, as do the DSP 112 and the DSP program/data memory 113. Also shown are DSP accelerators 114, including a Gaussian minimum-shift keying (the modulation scheme for GSM) accelerator and an equalizer accelerator.
D. Sleep Circuitry.
One significant challenge facing designers of mobile stations is conserving power. Because mobile stations are generally powered by batteries, mobile stations which consume available power quickly have a significant disadvantage. One manner in which power can be conserved is to introduce sleep circuitry into the mobile station. With sleep circuitry, when the mobile station is in idle mode (i.e., listening to a paging channel periodically, but otherwise taking no action), the control processor commands the mobile station to enter into a sleep mode to minimize power consumption. During sleep mode, the processors (i.e., the control processor and the DSP) are shut down, thereby conserving energy.
A problem with current sleep mode circuitry is that, while the mobile station is in sleep mode, the 13 MHz clock is still in operation. Because of the high frequency of the reference clock, significant power is consumed even during sleep mode.